RDNIS is correct - you use keyframes
More specifically, you use Bezier masks and keyframes -
On the event containing the character use open up Event Pan & Crop.
Check the tick-box at the bottom of the window labelled "Mask"
You can now draw a mask around the character.
Move forward a few frames and move/add/delete the anchor points to suit.
This will automatically create a new keyframe.
Repeat ad nauseum.

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Tim